The death rate from Covid-19 is more than 10 times greater than in Germany. Why does the death rate vary so much internationally?

In Italy, an epicentre of the new coronavirus outbreak, the death rate at the end of March stood at a sobering 11%. Meanwhile in neighbouring Germany, the same virus led to fatality rates of just 1%. In China, it was 4%, while Israel had the lowest rate worldwide, at 0.35%.

When the first Covid-19 case in Vò, Italy was confirmed, testing was rolled out to the entire village (Credit: theconversation news)

At first it can seem surprising that the same virus – which doesn’t seem to have mutated significantly as it has spread – can lead to such widely differing reported mortality rates. And even within one country, the rate appears to change over time. So what’s going on?

Covid-19 is markedly higher than elsewhere in the world, which is largely down to how cases are detected (Credit: Worldometer/BBC)

Several main factors account for much of the difference we’re seeing – and perhaps the most important come down to simply how we’re counting, as well as testing, cases.

Differing death rates

First, there is confusion about what people mean by “death rate”. This confusion can make countries’ numbers look vastly different, even if their populations are dying at the same rate.

There are, in fact, two kinds of fatality rate. The first is the proportion of people who die who have tested positive for the disease. This is called the “case fatality rate”. The second kind is the proportion of people who die after having the infection overall; as many of these will never be picked up, this figure has to be an estimate. This is the “infection fatality rate”.

In other words, the case fatality rate describes how many people doctors can be sure are killed by the infection, versus how many people the virus kills overall, says Carl Heneghan, an epidemiologist and director of the Centre for Evidence-Based Medicine at the University of Oxford; he is also a GP in recovery from a suspected Covid-19 infection.

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To see what a difference this makes, consider 100 people who have been infected with Covid-19. Ten of them have it so severely that they go into hospital, where they test positive for Covid-19. The other 90 are not tested at all. One of the hospital patients then dies from the virus. The other 99 people survive.

That would give a case fatality rate of one in 10, or 10%. But the infection fatality rate would be just one in 100, or 1%.

The lack of widespread, systematic testing in most countries is the main source of discrepancies in death rates internationally

So if some countries only test patients ill enough to go to hospital – and don’t test the less-ill (or even asymptomatic) Covid-19 patients who don’t get to hospital (which is what the UK is currently doing) – the death rate can appear higher than in countries where testing is widespread (such as Germany or South Korea).

Even if you’re careful to compare the same type of fatality rate across countries, it’s easy to see how testing more, or fewer, people would change the results.

In fact, the lack of widespread, systematic testing in most countries is the main source of discrepancies in death rates internationally, says Dietrich Rothenbacher, director of the Institute of Epidemiology and Medical Biometry at the University of Ulm in Germany.

As a result, the current figures are “not at all” directly comparable between countries, he says. This is because, to get an accurate figure across a population, it is necessary to test not just symptomatic cases, but asymptomatic people too. Having that data would give an accurate picture of how the pandemic is affecting whole populations, not just the sick.

“Currently we have a huge bias in the numbers coming from different countries – therefore the data are not directly comparable,” he says. “What we need to really have valid and comparable numbers would be a defined and systematic way to choose a representative sampling frame.”

The village of Vò in northern Italy is an example of why testing is important not just to get accurate data, but to contain Covid-19. When the first Covid-19 case in Vò was confirmed, testing was rolled out to the entire village of 3,300 people. The results showed that at the time of the “first case”, 3% of the village was already infected, but showing no or few symptoms.